Because of the continuing proliferation of wireless electronic systems installed on structures, such as cars, trucks, boats, aircraft, missiles, and human bodies, and because of the limited availability of surface area suitable for antenna mounting on these structures, there is a rapidly growing need in the industry to reduce the size and the number of antennas which are mounted on these structures. One approach to reduce the number of antennas is to implement a multimode, or multifunction, antenna, which can handle two or more wireless electronic systems concurrently. However, there are only a few multifunction antennas known in the industry today, and they are mostly of the type having high-directivity (i.e., high gain in particular directions). Furthermore, these few antennas are not conformal and low-profile, and therefore take up considerable space when mounted on a vehicle or structure.
To achieve multifunction operation, an antenna generally needs (1) a wide, though not necessarily continuous, frequency bandwidth, and (2) diversity of the radiation pattern and polarization. Very few antennas have these properties. Requirements (1) and (2) arise from the fact that different electronic systems operate at different frequencies and require different radiation patterns and polarizations.
The need for surface conformability, or the ability to shape an antenna to suit a particular application, is based primarily upon practical considerations, not upon the electrical performance of the antenna. More specifically, for aircraft or missiles, for instance, aerodynamic considerations dictate that an antenna have a low profile with little or no protrusion from or into the body of these devices. For the automobile, as another example, the need for protection against breakage, vandalism, damage in a car wash, etc., and the desire for privacy, security, or aesthetic appeal motivate car manufacturers to attempt to develop antennas which can be integrated into the rooftop structure as a hidden antenna.
As for conformability, very few antennas can claim this feature. A review of existing antennas in the art quickly reveals that there is no known antenna that has both surface conformability and a bandwidth of over 30%, except for the spiral-mode microstrip (SMM) antenna recently invented by J. J. H. Wang (also the inventor herein) and V. K. Tripp, as set forth in U.S. Pat. No. 5,313,216.
Although the SMM antenna of U.S. Pat. No. 5,313,216 is potentially capable of generating various modes, it is not a trivial matter to excite, extract, and multiplex one or more modes from a single SMM antenna. This problem could be handled by using a complex feed matrix, such as a Butler matrix, which is well known in the art, or the feed network originally designed for the cavity-loaded spiral antenna described in R. G. Corzine and J. A. Mosko, Four-Arm Spiral Antennas, Artech House, Norwood, Mass., 1990. However, the foregoing feed networks are undesirably complex, bulky, and expensive. Accordingly, a heretofore unaddressed need exists in the industry for improved methods for exciting, extracting, and generating various SMM antenna modes with much less complexity, requisite space, and expense as compared to the prior art.